ci: add Dependabot config

[iliassjabali]

Summary

• Adds .github/dependabot.yml covering all ecosystems present in the monorepo: github-actions, npm (root pnpm workspace), pip (control-plane, operator, sdk-python, sdk-langgraph), and docker (control-plane, operator, sidecar).
• Weekly cadence, with minor+patch updates grouped per ecosystem to minimize PR noise; majors stay individual for review.
• PRs labeled dependencies plus an ecosystem tag (ci, npm, python, docker) for easy filtering.

Test plan

• Merge to main
• Open Insights → Dependency graph → Dependabot and confirm all 9 entries show "Last checked" with no errors
• Within the first weekly cycle, verify grouped PRs appear with the expected labels

Replaces #43 (closed after branch rename from mystifying-driscoll → ci/dependabot-config).